The present invention relates to an apparatus for causing a portion of the compressed air from the compressor section to bypass a combustor in a gas turbine so that the bypassed air enters the hot gas flow path downstream of the combustor but upstream of the turbine.
A gas turbine is comprised of a compressor section that produces compressed air that is subsequently heated by burning fuel in a combustion section. The hot gas from the combustion section is directed to a turbine section where the hot gas is used to drive a rotor shaft to produce power. The combustion section is typically comprised of a shell that forms a chamber that receives compressed air from the compressor section. A plurality of cylindrical combustors are disposed in the chamber and receive the compressed air along with the fuel to be burned. A duct is connected to the aft end of each combustor and serves to direct the hot gas from the combustor to the turbine section.
In the past, a cylindrical collar, sometimes referred to as a "clam shell," was used to join the aft end of the combustor to the forward end of the duct. The collar was longitudinally split into two halves and joined along flanges. The collar encircled the aft end of the combustor and the forward end of the duct so as to join the two components together.
In order to control the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant, during the combustion process, it is sometimes desirable to cause a portion of the compressed air from the compressor section to bypass the combustors, especially during part-load operation. In the past, this has been accomplished by installing a butterfly type valve directly into the duct that directs the hot gas to the turbine so that a portion of the compressed air from the chamber bypasses the combustor and enters the hot gas flowing through the duct.
Unfortunately, this approach suffers from a variety of drawbacks. The duct must frequently be replaced because of the effects of thermal stress and corrosion. Hence, the incorporation of the butterfly valve directly into the duct increases the cost of maintaining the gas turbine. Second, introducing air directly into the duct at one localized spot can create distortions in the temperature profile of the hot gas flowing into the turbine section. Third, the butter fly valves are subject to leakage, resulting in a loss in thermodynamic performance when the bypassing of air is not desired.
It is therefore desirable to provide an apparatus for causing a portion of the compressed air from the compressor section to bypass the combustor and enter the hot gas flow path downstream of the combustor that will be durable, prevent distortions in the gas temperature profile, and prevent unwanted leakage of air into the hot gas flow path.